Chapter 16 Notes - Chapter 16 Notes-The Cytoskeleton...

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Chapter 16 Notes-The Cytoskeleton Functions of the cytoskeleton : -establish cell shape -mechanical strength -cell movement -chromosome separation -intracellular transport of organelles Actin filaments - determines the shape of cell’s surface and involved in locomotion Microtubules -determine the position of organelles and direct intracellular transport Intermediate filaments - provide mechanical strength Motor proteins - convert energy of ATP hydrolysis to move organelles along filaments or filaments themselves Microtubules rearrange to form the mitotic spindle during cell division Actin filaments form a contractile ring during cytokinesis which pinches the dividing cell in two. Actin filaments also contribute to cell-cell junctions. Intermediate filaments are anchored to desmosomes and hemidesmosomes. Microtubules provide a coordinate system or polarity to direct components to proper locations in the cell. Actin filaments are formed from actin subunits (ATP binding protein) Microtubules are formed from tubulin subunits (GTP binding protein) Cytoskeletal polymers or filaments are formed from multiple protofilaments which twist around one another in a helical lattice. Intermediate filaments-subunits form strong lateral bonds Microtubules-subunits held together by strong longitudinal bonds Nucleation (lag phase) is the rate-limiting or the slow step in the formation of a polymer. Nucleation- initial process of forming a nucleus or aggregation of subunits Critical concentration (Cc) – the concentration of free subunits when the rate of subunit addition to the filament ends balances the rate of subunit disassociation Lag phase in filament growth is eliminated if preexisting subunits are present
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Microtubules -the subunit is a tubulin heterodimer of alpha-tubulin monomer and beta-tubulin monomer -each monomer has a binding site for GTP -GTP bound to alpa-tubulin is trapped and is never hydrolyzed -nucleotide on beta-tubulin can be in GTP or GDP form -beta-tubulin on top of alpha-tublin (strong binding) -lateral contacts are alpha-alpha or beta-beta Microtubule is a hollow cylindrical structure built from 13 parallel protofilaments Microtubule has a structural polarity because alpha tubulin at one end and a beta-tubulin at the other end Actin filaments -globular monomer with an ATP binding site -two parallel protofilaments in a twisted right-handed helix Structural polarity -plus end: growth and shrinkage occurs the fastest -difference in the rates of growth is because of changes in the conformation of each subunit as it enters the polymer Microtubules: alpa-subunits at minus end; beta-subunit at plus end. Actin filaments: ATP-biding cleft points toward the minus end
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Chapter 16 Notes - Chapter 16 Notes-The Cytoskeleton...

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